1. Field of the Invention
This invention relates to large-area, non-polar and semi-polar gallium nitride (Al, Ga, In)N substrates useful for producing optoelectronic devices (such as light emitting diodes (LEDs), laser diodes (LDs) and photodetectors) and electronic devices (such as diodes and field effect transistors (FETs)) composed of III-V nitride compounds, and to methods for producing such articles.
2. Description of the Related Art
Group III-V nitride compounds such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN, and AlGaInN, are direct bandgap semiconductors with bandgap energy ranging from about 0.6 eV for InN to about 6.2 eV for AlN. These materials may be used to produce light emitting devices such as LEDs and LDs of short wavelength in the green, blue and ultraviolet (UV) spectra, as well as high-frequency, high power RF devices, and device power electronics, such as diodes and field effect transistors (FETs).
GaN-based semiconductor materials and devices have historically been developed through crystal growth in the [0001] direction (c-plane). The lack of mirror-like (or two-fold) symmetry of the hexagonal crystal structure gives rise to spontaneous polarization in the [0001] direction. Additionally, strain in lattice mismatched hetero-epitaxially grown device layers result in piezoelectric polarization. These polarization effects result in large built-in electric fields, hampering the performance of nitride-based devices. In optical devices, the built in polarization field results in charge separation within quantum wells and decreases the recombination efficiency of electron-hole pairs and red-shifts the emission wavelengths. For microelectronic devices, the spontaneous and piezoelectric polarization allows for the accumulation of a very high density of sheet charge (nd) in the conducting channel of GaN-based HEMTs; however the surface of the device requires the appropriate passivation and is sensitive to the stress induced by passivation and thermal effects. As a result, researchers have investigated methods to eliminate the built-in fields of devices grown in the [0001] direction.
Recently, attention has been paid to the development of nitride epitaxial layers and heterostructures with non-polar and semi-polar crystal orientations. GaN-based quantum structures grown along non-polar directions, such as the [1100] (resulting in m-plane surfaces) and [1120] (resulting in a-plane surfaces) have been shown to be free of the aforementioned polarization effects. Additionally, certain orientations of the GaN crystal structure, in particular the [1101] and [1122] have been identified as orientations with low spontaneous polarization relative to the [0001] direction, and have been labeled as semi-polar orientations. Using heteroepitaxially grown non-polar and semi-polar thin films and devices fabricated thereon, higher internal quantum efficiencies and lower EL peak sensitivity for LEDs have been demonstrated. Initial results in this emerging technological area show great promise for non-polar and semi-polar substrates in impacting a number of commercial applications, such as laser diodes, visible and UV LEDs, and high power electronic devices. One of the current limitations in developing non-polar and semi-polar GaN-based devices is the availability of high-quality, large area substrates, which is due to limitations in crystal growth technology.
In order to achieve large area, low defect density non-polar substrates, different growth techniques, substrate materials, and layer orientations have been investigated. The majority of these approaches involve the heteroepitaxial growth of non-polar GaN on non-native (non-GaN or other nitride) substrates. The published research studies are consistent that, in addition to the well known problems for heteroepitaxially grown nitrides in the [0001] direction on sapphire and SiC, such as high dislocation density and strain induced effects, the material grown in [1120] direction on both (1102)-plane (r-plane) sapphire and (1120)-plane (a-plane) SiC, the non-polar GaN material includes a high density of stacking fault (SF) defects and in-plane anisotropy of all the materials properties. Lower mismatch substrates such as spinel (MgAl2O4) and lithium aluminum oxide (LiAlO2) have been suggested with the hope of enabling lower defect density, yet similar structural and impurity related problems have been observed. The well known approach for epitaxial lateral overgrowth (ELOG), employing a selective-growth stripe pattern, has also been used and has been found to improve the morphology and to reduce significantly the defect density, but still the optical properties have been defect dominated. Practically speaking, the ELOG approach has a number of implementation difficulties, including film cracking, anisotropic growth modes in different crystalline directions, stress and crystalline tilt across wing regions and uniformity of coalescence across large areas. Bearing these challenges in mind, an approach to fabricating GaN crystals with selected non-polar crystal orientations of large size and low defect density is desired.
We have investigated and developed the hydride vapor phase epitaxy (HVPE) growth technique for fabricating non-polar GaN substrates. In one example, HVPE layers grown on sapphire in the [0001] direction were grown to ˜1 cm and sliced into non-polar, freestanding bulk GaN substrates. The materials properties of these substrates were extensively characterized using a variety of techniques, and were compared to non-polar and semi-polar GaN layers and substrates grown on non-native substrates. This approach has several advantages over the direct growth of non-polar GaN material. First, the resulting GaN crystal enables one to select the desired non-polar or semi-polar orientation, enabling on-demand crystal orientation for the substrate. Second, preliminary characterization results show the material uniformity and the defect density are significantly improved over heteroepitaxially grown material. The success of this approach depends on the ability to grow thick HVPE layers in the [0001] direction and this capability has been demonstrated for fabrication of [0001] substrates for optoelectronic and microelectronic applications. Increasing the size of non-polar and semi-polar substrates using this technique requires extended growth in the [0001] direction, i.e. 2 inches or more for 2 inch diameter non-polar substrates. According to the present teachings as described below, an alternative method to providing large area, low defect density non-polar and semi-polar substrates is to use a native (i.e. GaN) seed crystal for further growth of bulk GaN material and to then expand the size of the seed crystal laterally to obtain larger substrates.